FIG. 1 illustrates a conventional floor beam 1 for an aircraft fuselage. The beam 1 comprises an I-section with a stiffened shear web 2 extending between upper and lower flanges 4, 6. The shear web 2 is provided with a regularly spaced series of identical oval weight-saving and system cut-outs 8. A number of stiffeners 9 are provided along the length of the web between the cut-outs to increase the amount of load which can be supported by the beam 1.
As most clearly seen in FIG. 2 (“Airframe Structural Design”, by Michael C. Y. Niu, 2nd Edition, Hong Kong Conmilit Press Ltd.), a number of these floor beams 1 typically extend across the width of the fuselage to support a fuselage floor. The ends 7 of each beam 1 are bolted to a respective fuselage frame 10. Each beam 1 is also supported vertically by a pair of floor beam support struts 11 which are joined to a corresponding pair of clevis fittings 12 which extend down from the lower flange 6. Additionally, a number of seat rails 13, which extend parallel to the longitudinal axis of the fuselage, are attached to the upper flanges 4 of the floor beams 1.
The seat rails, along with cabin items, such as passengers, monuments, partitions etc carried by the fuselage floor, exert a loading (down, up, longitudinal and lateral) on the upper flanges 4 of the floor beams 1. The floor beam support struts 11 exert a relatively concentrated upward vertical reaction load on the lower flanges 6 via the clevis fittings 12. The regions of the shear web 2 adjacent the fittings 12 are particularly critical areas of the beams 1 as they need to support the concentrated shear loads exerted by the floor beam support struts 11.
To avoid extreme local bending of the lower flange 6, the shear web 2 is locally reinforced with extra stiffeners around the fittings 12 (or alternatively by locally thickening the shear web 2). This local reinforcement also ensures a gradual shearing of the concentrated vertical load into the beam. However, the extra reinforcement adds significant weight to the design.
As the aircraft fuselage can contain upwards of 60 of these floor beams, any reduction in the weight of an individual beam 1 can lead to significant overall weight savings.
Optimisation of the conventional floor beam design is typically performed by varying the width/thickness of the upper/lower flanges, by varying the width/thickness of the vertical stiffeners and by varying the thickness of the shear web. The number of vertical stiffeners and the distance between them, as well as the size and shape of the cut-outs 8 can also be varied.